In the graphic arts industry, clear plastic films or sheets made of various polymers such as polyesters, polycarbonates or polyvinyl chloride are often used as the substrate support for signs and the like. The films are typically printed on with various designs that may be utilized to provide advertising or other information. The preferred printing materials are ultraviolet curable inks or water-based inks since they are typically solvent free. Of course, solvent-based inks are still utilized. Printing techniques vary depending on the particular application and printing operation.
Adhesion problems exist with certain inks, particularly the ultraviolet curable inks, when printed on substantially amorphous polymeric substrates. Adhesion of the ink refers to the bond between the ink and the substrate. Ideally, the adhesion is such that there would be 100 percent retention of the ink to the substrate.
Blocking, a common problem in the film industry, relates to properties of the ink. Blocking occurs when a printed film that has been rolled for shipping is exposed to heat and pressure. The exposure softens the resin in the printing ink allowing the ink to adhere to the backside of the adjacent film. When the blocked films are separated, damage to either the ink film, substrate or both can occur. To solve blocking, U.S. Pat. Nos. 5,006,598 and 5,041,489 disclose ink compositions comprising linear, water dispersible copolyesters containing sulfoisophthalic acid moieties. Blocking and adhesion differ since the former deals with the cohesive properties of the ink only and the latter deals with the bonding properties between the ink and a polymeric substrate.
Another problem with substantially amorphous substrates, in particular poly(ethylene terephthalate) copolyester containing 1,4-cyclohexanedimethanol, is that the substrates when contacted with other substrates stick together, i.e. the substrates have poor slip characteristics. Often a piece of tissue-like paper is used to keep the surfaces separate.
Thus, there exists a need in the art for a primer to the surface of polymeric films to provide improved adhesion of printing inks to the polymeric films and improved slip characteristics as measured by a decrease in the static or kinetic coefficient of friction. Accordingly, it is to the provision of such that the present invention is primarily directed.